Preadjusted and integrated "straight-wire" orthodontic bracket systems are well known, in which all the necessary angles and planes of movement--commonly referred to as "tip," "torque," "in-out" and "anti-rotation"--are manufactured directly into the brackets. These straight-wire appliances, when properly placed on teeth, are designed to allow the force and resilience of unbent archwires to work with the preadjustments to guide the teeth into ideal positions.
Contemporary appliance system designers strive to identify the proper position desired for each tooth at the conclusion of fixed appliance therapy (mechanotherapy), so that the individual brackets can be pre-built to provide, in conjunction with an archwire, theoretically appropriate alignment forces to the respective teeth. For example, if a designer desires seven degrees of crown torque, then a seven degree torque angle has typically been built into the individual edgewise bracket by well-known methods. The amount of torque actually expressed by a bracket, however, is dependent upon the wire-to-slot deviation angle--referred to as slot "play." Some play generally exists between any archwire and an edgewise bracket slot, even if the archwire is considered "full-size" relative to the slot. This play reduces the actual torque provided by the system from its full and theoretically correct expression.
To compensate for inadequate torque built into many preadjusted bracket systems, positive (+) crown torque is commonly added to the upper anterior segment of rectangular or square archwires late in treatment. Similarly, it may be desirable to apply a small amount of lingual or negative (-) crown torque to the lower anterior (i.e., central and lateral) teeth early in treatment to maintain their "upright" orientation.
These anterior-specific torque treatments are handled in various ways. Orthodontists most often add positive crown torque to the upper anterior segment by manually bending the torque into the anterior portion of individual stainless steel archwires. Titanium-based alloy archwires, particularly nickel titanium (nitinol) archwires, are generally considered to be ideal for orthodontics, especially in comparison to stainless steel, largely due to the low force these wires impart to crowns, coupled with their outstanding shape memory. This "superelasticity" is more biocompatible, meaning that there is less chance of damaging the dentition by moving crowns too quickly. Titanium-based wires also provide more complete correction without the need to constantly change archwires due to permanent deformation. It is virtually impossible, however, for an orthodontist to manually bend torque into titanium-based archwires due to this outstanding resistance to permanent deformation. This is why, for the critical final torque moments that are commonly bent into archwires, orthodontists are generally limited to the high forces and low shape memory of stainless steel, as opposed to the far more comfortable and biocompatible forces of nickel titanium or other titanium-based alloys.
For purposes of this invention, it will be understood that the term "titanium-based alloy" is intended to include nickel titanium alloys (with or without other elements, such as copper, columbium, iron or aluminum), beta titanium alloys and near-beta titanium alloys.
Manually bending torque into individual stainless steel archwires is a complex procedure that requires tremendous clinical skill and experience. Even substantial skill and experience do not assure an orthodontist that the desired torque will be imparted to the archwire or that the desired correction will occur precisely for each patient. Moreover, since an orthodontist is almost always directly responsible for manually torquing individual archwires, a great deal of "chairtime" is required.
In regard to applying lingual crown torque to the lower anterior teeth early in the treatment process, this is commonly attempted by using lower anterior brackets which have small amounts of torque built into them. Since small archwires allow for too much "play" in the bracket slots to express low levels of built-in bracket torque, "full-size" archwires are required if there is to be any meaningful expression of the desired lingual torque. Indeed, orthodontists typically increase bracket torque by using a larger bracket wire. Unfortunately, using a large edgewise wire early in treatment is not recommended, since the force provided is likely to be excessive and may cause biocompatibility problems (e.g., an overall reduction in the length of the root of the tooth, referred to as root "resorption"). Likewise, building more torque into the lower anterior brackets to compensate for the archwire play of small wires is not recommended, since problems could arise later in treatment when larger edgewise wires are employed, as they would then begin to express excessive amounts of lingual crown torque.
Because of these various difficulties in anterior-specific torque treatment, pretorqued archwires have been developed in which torque is built directly into the wire during manufacture. Pretorqued wires save valuable chairtime over manual wirebending (particularly since application of them can often be delegated to an orthodontic assistant as a routine wire change), they have greater biocompatibility (if titanium-based wire is used) and they theoretically provide more accurate results.
Currently, two types of pretorqued archwires are available to the orthodontist. One is a preformed stainless steel archwire. These wires have the drawbacks, however, of high force, resultant poor biocompatibility and low shape memory. Also, the built-in torque in all presently known steel versions extends through both the anterior and the posterior segments of the wire. It is not common to add torque to the upper posterior teeth, and it is usually very undesirable to increase torque in the lower posterior teeth. Consequently, the current pretorqued stainless steel archwires are seen as providing little improvement, if any, over the common art of manually bending wires, and they have had very little success in the marketplace.
Pretorqued nickel titanium archwires have also been introduced, but they too have achieved little commercial success since they do not meet certain critical criteria. Specifically, the built-in torque does not appear to be limited to the anterior segment, as is desired for proper treatment. Rather, positive torque appears to extend into the posterior area, where negative crown torque, if any, is most desirable. The transition from passive to torqued (active) wire should ideally occur in the area between the lateral and cuspid brackets (distal to the lateral bracket and mesial to the cuspid bracket), but in the available nitinol pretorqued archwires this transition area is very broad, with the result that too little torque force is imparted to the lateral teeth and/or an undesirable (positive) torque force is imparted to some of the posterior teeth.
U.S. Pat. No. 5,722,827, assigned to ORTHO SPECIALTIES.TM. discloses a torqued archwire for use in an edgewise orthodontic bracket system. The archwire is substantially rectangular in cross-section, is made of titanium-based alloy and has a plurality of segments, including an anterior segment which is sized to cooperate with brackets attached to a person's central and lateral teeth. Torque is built into this anterior segment, being maximized in a range of about 13-20.degree. at the centerpoint and adjacent the central teeth brackets and then decreasing continuously along the remaining length of the segment. Extending from the respective ends of the anterior segment is a pair of transition segments, in which the torque built into the archwire diminishes to zero. The lengths of the transition segments are such that these "zero" points occur, in use, between the corresponding adjacent lateral and cuspid brackets. Posterior segments of the archwire, which extend distally from the transition segments, generally include no built-in torque.
It has been discovered that torque is sometimes desirable at values greater than can be attained with the system disclosed in U.S. Pat. No. 5,722,827. In efforts to achieve greater torque, orthodontists have used archwire of a larger size, reducing the amount of angular "play," according to the known practice mentioned above.